Kinojo Castle

Overview and Location

**Kinojo Castle** (Kino-jō; "Demon Castle") is an ancient mountain fortress located in Sōja City, Okayama Prefecture, on the summit of Mount Kijō (approximately 397 meters above sea level). Encircled by approximately 2.8 kilometers of walls, it is one of Japan's most significant ancient mountain castle (kodai yamashiro) sites. Over 80 percent of the walls are constructed as rammed-earth ramparts (hanchiku dōrui) — roughly 7 meters wide and 6–7 meters high — supplemented by stone-faced sections. Four gates, six water gates, corner towers (sumiyagura), foundation stones of buildings, and evidence of a smithing workshop have been identified through ongoing excavation. The site was designated a National Historic Site ("Mt. Kijō") in 1986 and was selected as No. 69 among Japan's 100 Famous Castles in 2006.

7th-Century Korean-Style Mountain Castle

Kinojo is classified among the ancient "Korean-style mountain castles" (Chōsen-shiki yamashiro) constructed in Japan during the latter half of the 7th century. The site appears in no historical chronicles (Nihon Shoki or Shoku Nihongi), and no definitive primary sources identify its builders or construction date. However, ongoing archaeological work by the Sōja City Board of Education and the Okayama Prefectural Board of Education has assigned the remains to the Asuka IV period (the fourth quarter of the 7th century through the early 8th century), making construction in the second half of the 7th century the most widely accepted scholarly conclusion.

This period immediately followed the catastrophic Japanese defeat at the Battle of Hakusukinoe (663 CE), in which a Yamato-Paekche coalition force was routed by Tang Chinese and Silla Korean combined forces. In response, the Yamato court urgently strengthened its defensive posture against possible continental invasion, constructing a chain of mountain fortresses from Tsushima Island and northern Kyushu through to the home provinces (Ōno-jō, Kii-jō, Kaneda-jō, and others). Kinojo is thought to have formed part of this defensive network, with evidence suggesting the involvement of continental technicians — likely refugees from Paekche — in its construction.

Structural Features

The castle walls follow the ridgeline of Mount Kijō in a roughly symmetrical east-west, north-south configuration over 2.8 kilometers. The dominant construction technique is hanchiku (rammed earth in horizontal layers), with stone-facing incorporated in specific sections. The West Gate is the largest of the four gates and has been restored with a roofed gate structure as part of the public archaeological park. Six water gates managed drainage from the mountain interior while preventing unauthorized entry.

The site contains what researchers have identified as Japan's earliest known stone-paved surface (shikishi iseki), evidence of sophisticated continental construction techniques. Within the enclosure, the foundations of seven buildings have been mapped, along with the traces of a smithing workshop (twelve furnace pits), storage facilities, and a signal fire platform (hōkadai).

The Ura Legend and the Origins of the Momotarō Tale

Kinojo is deeply intertwined with the Ura legend (Ura Densetsu). In this tradition, Ura was a royal descendant from the Korean peninsula (Paekche) who established himself at Kinojo, controlled the Kibi region, and preyed upon passing ships. To suppress him, Ōkibitsu-hiko-no-Mikoto — said to be a grandson of Yamatotakeru — was dispatched and defeated Ura in combat. This mythological conflict is associated with the Kibitsu-hiko Shrine and is widely regarded as one of the prototypical sources for the Momotarō (Peach Boy) folktale.

Academically, the Ura legend is interpreted as a narrative reflecting the encounter and integration between immigrant communities (Paekche and Silla technicians) and local Kibi powers — a reading consistent with evidence for continental involvement in the castle's construction.

Smithing Workshop and Arms Production

One of Kinojo's most archaeologically significant features is the smithing workshop identified within the enclosure. Twelve furnace pits (kajiro) have been excavated, indicating organized iron production within the castle. Recovered artifacts include nails, yariganna (a type of spokeshave plane used in woodworking and metal finishing), and whetstones, suggesting that swords, weapons, and agricultural and construction implements were produced and maintained on-site as part of the castle's operational infrastructure.

This evidence reveals Kinojo not merely as a military fortification but as a multi-functional complex with integrated production facilities. The discovery of an organized smelting and smithing operation within a 7th-century mountain castle is an exceptionally important finding for understanding the relationship between ancient Japanese ironworking and military infrastructure, and is of considerable interest to both archaeologists and students of Japanese sword history.

Abandonment and Legacy

Researchers generally characterize Kinojo as a castle that "fulfilled its mission within a very short time." Evidence suggests the site's primary defensive function ceased around the beginning of the 8th century. The principal cause was likely the easing of tensions between Japan and the Tang-Silla alliance in the decades following Hakusukinoe, reducing the perceived threat of continental invasion.

After its operational abandonment, Mount Kijō was venerated as a sacred mountain of the Kibi region, and in later centuries the "demon's mountain" became the setting for numerous local legends. In modern times, Sōja City has developed the site under the concept of an "outdoor museum of history and nature," reconstructing the West Gate, corner towers, and sections of the earthen walls for public appreciation.

Current State and Highlights

Visitors to the restored site can walk along sections of the rammed-earth ramparts, view the reconstructed West Gate and corner tower, and enjoy panoramic views of the Kibi Plain from the summit — on clear days, the Seto Inland Sea is visible in the distance. The Kinojo Visitor Center at the base of the mountain exhibits excavated artifacts and explains both the archaeological findings and the associated Ura legend, providing a comprehensive introduction to the site's dual significance. Access is via a mountain trail (approximately 60 minutes on foot from Hattori Station on the JR Kibi Line), or by taxi from central Sōja. Parking is available near the summit.